You then have 2 problems: (a) What email clients will support it and (b) con artists will just put big green borders around their spam emails.
(Edit: It is also conceivable that a client could put another prominent border around HTML mail, to mitigate the issue somewhat.)
You then have 2 problems: (a) What email clients will support it and (b) con artists will just put big green borders around their spam emails.